this accuracy is quite interesting because versions given by program libraries, most often use hasting approximation, which is a polynomial approximation accurate to within 7.5x10^-8 (worst case around 0.717). clearly this model does not use this same approximation internally. hasting approximation is used by the hp29c program library and also turns up in casio models like the fx-201p and later models inheriting the same library.
the inverse operation Q^-1 performs an iteration. you can see the digits dance whilst this calculates for several seconds. it too is accurate.
shown here is how to dismantle a spice calculator.